Failure of insulin secreting pancreatic islet cells is a hallmark of both type1 and type 2 diabetes, leading to this laboratory's intense focus on molecular mechanisms regulating pancreatic cell development and function. We previously identified Pcif1 in a yeast two hybrid screen for partners of Pdx1, a homeodomain transcription factor and human diabetes gene pivotally positioned in the transcriptional hierarchy governing the development of cell mass, the function and survival of adult cells, and the ability of adult cells to compensate for increased metabolic demand imposed by insulin resistance. During the previous funding period we determined that Pcif1 acts at least in part through its role as a substrate adaptor to recruit Pdx1 into a cullin3-based complex for ubiquitination and proteasomal degradation, thereby implicating Pcif1 as a target for regulating Pdx1 levels. Indeed, Pcif1 loss of function in cells elevates Pdx1 protein and stimulates expression of important Pdx1 transcriptional targets, including insulin, MafA and Glut2. In vivo genetic disruption of Pcif1 reveals roles in regulating adult cell turnover and function and in endocrine lineage allocation during embryonic development, through Pdx1-dependent and, likely, Pdx1-independent mechanisms. We hypothesize that Pcif1 critically regulates embryonic cell formation and the adult cell cycle through its ability to modulate the levels/functions of key transcriptional regulators. This hypothesis will be tested in two aims: Aim 1. To determine how Pcif1 regulates the turnover of adult cells, and Aim 2. To determine how Pcif1 influences the emergence of the cell lineage during embryonic pancreas development. Insights gained from the proposed experiments will provide a conceptual scaffold for the development of targeted therapeutics that interfere with Pcif1 expression or its interactions with critical substrates in the cell to influence cell mass and cell function, with potential application to approaches involving stem cell differentiation to cells, cellular reprogramming of cells from other mature cell types, and for approaches to promote the function and regeneration of endogenous beta cell mass in patients with diabetes.